The present invention relates to a biaxially oriented laminated polyester film. More specifically, it relates to a biaxially oriented laminated polyester film which has excellent winding properties and handling properties and can provide excellent electromagnetic conversion characteristics when used as a base film for high-density magnetic recording media, preferably for digital magnetic recording media, particularly for ultra high-density magnetic recording media.
A biaxially oriented polyester film typified by a polyethylene terephthalate film is used for various purposes, particularly as a base film for magnetic recording media, because of its excellent physical and chemical properties.
Along with recent efforts made to increase the density and capacity of a magnetic recording medium, a flatter and thinner base film is desired. Particularly, a multi-layer metal-type tape having performance equivalent to a deposited tape has been developed and a demand for a flat base film has been arising. However, when the surface of a base film is flattened to maintain excellent electromagnetic conversion characteristics, its slipperiness or air squeezability deteriorates. As a result, when the film is to be rolled, it is apt to have wrinkles or debris, which make it extremely difficult to roll the film. When the slipperiness of the base film is low in the film processing step, the friction of the base film with a metal roll in contact with the base film increases, whereby the film is wrinkled, so that a magnetic layer cannot be formed nicely or that the film cannot be calendered well.
To improve the slipperiness of a polyester film, the following methods are employed: (1) one in which inert particles are precipitated from a catalyst residue in a raw material polymer in the production process and (2) one in which the surface of a film is made uneven, for example, by adding inert particles. Generally speaking, the greater the size or content of particles contained in the film the more the slipperiness of the film is improved.
Meanwhile, as described above, the surface of a base film is desired to be as even as possible in order to improve electromagnetic conversion characteristics. When a magnetic recording medium is formed from a base film having a rough surface, the unevenness of the surface of the base film is reflected on the surface of a magnetic layer after the formation of the magnetic layer, thereby deteriorating electromagnetic conversion characteristics. In this case, the larger the size and content of particles contained in the base film the greater the surface roughness of the base film becomes, thereby deteriorating electromagnetic conversion characteristics.
To improve both slipperiness and electromagnetic conversion characteristics which are antipodal properties, there is a widely known means of producing a laminated film having an even surface on which a magnetic layer is formed for improving electromagnetic conversion characteristics and an opposite rough surface for improving slipperiness.
JP-A 3-90329 discloses a biaxially oriented thermoplastic resin film formed by laminating a film consisting essentially of a thermosetting resin A and inert particles A and B and having a thickness of 0.01 xcexcm or more on at least one side of a film consisting essentially of a thermoplastic resin B by coextrusion. In this case, the average particle diameter of the inert particles A is 0.1 to 10 times as large as the thickness of the film of the thermoplastic resin A and the inert particles B are agglomerates of primary particles having a smaller average particle diameter than the average particle diameter of the inert particles A and no directionability.
However, even when the above biaxially oriented laminated polyester film is used and the surface opposite to the magnetic layer-formed surface (to be referred to as xe2x80x9crough surfacexe2x80x9d hereinafter) is roughened, a lubricant exerts an influence upon the magnetic layer-formed surface, undulates the even surface and deteriorates the flatness of the even surface according to the quantity, type and size of the lubricant to be added to the rough surface, because the thickness of the base film is very small.
Particularly, in the latest high-density magnetic recording medium, a flatter magnetic layer is desired, a metal calender having a high linear pressure is used, and the projection of protrusions from the rough surface has a greater adverse influence upon the surface properties of the even surface.
To reduce the projection of protrusions onto the even surface from the rough surface, there have been proposed a method for reducing the particle diameter of a lubricant contained in the rough surface and a method for containing a small amount of a lubricant having a large particle diameter in the rough surface. However, in the former case, sufficient air squeezability cannot be obtained because the height of a protrusion formed is low whereas in the latter case, sufficient film slipperiness cannot be obtained because the density of protrusions formed is low. When the film is rolled, its surface has vertical wrinkles in the former case and debris in the latter case, thereby making it impossible to obtain a sufficiently high product yield.
Meanwhile, to improve electromagnetic conversion characteristics, a film having a more even surface for a magnetic layer is desired and there is proposed a flat layer containing substantially no lubricant. In this case, the running properties of the even surface in the tape-processing step are unsatisfactory, wrinkles appear on the film in that step, and the product yield is greatly reduced.
It is an object of the present invention to provide a biaxially oriented laminated polyester film which has excellent film winding properties and handling properties and can provide excellent electromagnetic conversion characteristics when used as a base film for high-density magnetic recording media, preferably for digital magnetic recording media, particularly for ultra high-density magnetic recording media.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, the above objects and advantages of the present invention are attained by a biaxially oriented laminated polyester film comprising:
(A) a first polyester layer which comprises a first aromatic polyester containing inert fine particles and has a surface roughness of 4 to 15 nm, the inert fine particles consisting of first inert fine particles having an average particle diameter of 0.2 to 0.4 xcexcm and second inert fine particles having an average particle diameter which is 0.1 to 0.3 xcexcm smaller than that of the first inert fine particles, and the contents of the first inert fine particles and the second inert fine particles being 0.05 to 0.4 wt % and 0.1 to 0.8 wt % based on the first aromatic polyester, respectively; and
(B) a second polyester layer comprising a second aromatic polyester which may contain inert fine particles and having a surface roughness of 0.5 to 3.5 nm, the inert fine particles being third inert fine particles having an average particle diameter of 0.05 to 0.2 xcexcm and the content of the third inert fine particles being 0 to 0.1 wt % based on the second aromatic polyester,
(C) the friction coefficient between the first polyester layer and the second polyester layer being 0.5 or less.